When positioning knee implants, an implant should be attached to the remaining bone in such a way that excessively high strains or strains on one side which can lead to a sensation of pain do not arise later, even during movement. In particular, the patella should be correctly positioned in order to be able to optimally configure its course when the implant is inserted.
In order to take these ancillary conditions into account when planning the implant, a common practice is to perform so-called patella tracking in a very basic embodiment. When “tracking” or “motion capturing” is mentioned in the present text, this then means, in very general terms, tracking the movement of a body with the aid of movement detection means. Thus, a body, or active emitters or passive reflectors attached to the body, can be tracked in order to be able to detect their location at a particular point in time (pre-/intra-/post-operative) and/or their trajectory and thus also the location or trajectory of the body. In particular, passive marker reflectors, which reflect infrared light, can be detected by two infrared light cameras, whereby spatial locations and/or spatial movements of the body can be determined by stereoscopic observation.
In conventional movement detection for the patella as noted above, reference arrays (arrays of, for example, three of the markers mentioned above) are attached to the patella and to the femur and possibly also the tibia, all three bones are registered, and a single point or just a few points on the patella are tracked in order to detect their trajectory relative to the femur.
The disadvantage of these known methods is simply their low yield of information on the relative movement and contact of individual parts of the joint which may be later expected. Point-by-point tracking or motion capturing for individual points on the patella is insufficient for a realistic simulation of the end conditions and therefore also insufficient for optimum planning.